It is conventional in paint production to grind the pigments to be used in the paints with part of the ultimate binder of the paint system or with special binders referred to at times as paste resins. The so-called milling pastes which result are then combined with the rest of the binder. It is evident that the grinding vehicle preferably is a special resin which has an exceptional pigment-wetting capacity. Furthermore, the grinding vehicle should exhibit good compatibility with the main binder; and it should not, at the required level, substantially detract from the properties of the main binder. Also, the grinding vehicle should have a relatively low viscosity.
In the operation of a CED bath, it has been shown efficient to replenish the bath by adding the bath material as two separate components, that is a high-pigmented color component and a clear varnish component. In this way monitoring of the bath, which can be done at times automatically, is substantially alleviated.
European Patent Application Nos. 0 107 088, 0 107 089, and 0 107 098 disclose synthetic resins which can be used as binders for pigment pastes based on quaternized monoepoxides. These products have the disadvantage in that they reduce the maximum deposition voltage (rupture voltage) of the paint system, that is of the diluted blend of the pigmented component and the clear varnish component, which in turn leads to a reduction of the throwing power, i.e., the coating of shielded parts of the object.
Binders carrying oxazolidine groups are disclosed in EP-B1 00 28 402, EP-A2 00 76 955, and AT-PS 372,689. In many cases the binders disclosed are recommended for the formulation of pigment pastes. However, due to their composition, such products cannot be pigmented in a binder/pigment ratio higher than about 2 parts-by-weight of binder to 3 parts-by-weight of pigment normal for CED coating because this would lead to pigment sediments and agglomerates in the ED bath which normally has a low solids content. In practice, this effect is reflected in a faulty coating, particularly on the horizontal surfaces of the object. The products disclosed in EP B1 00 17 428 have the same deficiency discussed above, and it is impossible to obtain flawless films with highly pigmented pigment pastes.
In order that a product is suitable for use as a pigment paste resin, the product must have a low viscosity and the solubility of the neutralized pigment paste in the bath material must be good in order to achieve a quick homogenization of the paint bath.